U.S. patent number 5,371,570 [Application Number 08/000,157] was granted by the patent office on 1994-12-06 for refractive/diffractive optical system for broad-band through-the-lens imaging of alignment marks on substrates in stepper machines.
This patent grant is currently assigned to The University of Rochester. Invention is credited to G. Michael Morris, Yasuhiro Yoshitake.
United States Patent |
5,371,570 |
Morris , et al. |
December 6, 1994 |
Refractive/diffractive optical system for broad-band
through-the-lens imaging of alignment marks on substrates in
stepper machines
Abstract
A chromatic aberration-corrected optical system for broad-band
through-the-lens (TTL) imaging and position detection of alignment
marks deposed on a substrate located at the exposure plane of an
exposure apparatus, for example, a stepper machine, uses a first
projection lens capable of focusing a first broad-band alignment
illumination and a second exposure illumination onto the substrate.
A second achromat lens and a third refractive/diffractive hybrid
lens are configured and designed to provide, in conjunction with
the first projection lens, longitudinal and lateral chromatic
aberration correction over a wavelength range from about 550-650 nm
of the broad-band alignment illumination.
Inventors: |
Morris; G. Michael (Fairport,
NY), Yoshitake; Yasuhiro (Yokohama, JP) |
Assignee: |
The University of Rochester
(Rochester, NY)
|
Family
ID: |
21690183 |
Appl.
No.: |
08/000,157 |
Filed: |
January 4, 1993 |
Current U.S.
Class: |
355/53; 355/41;
355/68; 355/71; 359/565 |
Current CPC
Class: |
G02B
27/0043 (20130101); G02B 27/4211 (20130101); G02B
27/4222 (20130101); G03F 9/70 (20130101) |
Current International
Class: |
G02B
27/00 (20060101); G03F 9/00 (20060101); G03B
027/42 () |
Field of
Search: |
;355/41,53,68,71 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Broadband Imaging with Holographic Lenses-Optical Engineering Jun.
1989 vol. 28 No. 6 pp. 592-598..
|
Primary Examiner: Gellner; Michael L.
Assistant Examiner: Malley; Daniel P.
Attorney, Agent or Firm: Lukacher; M.
Claims
We claim:
1. A chromatic aberration-corrected optical system for broad-band
through-the-lens imaging and position detection of alignment marks
disposed on a substrate located at the exposure plane of an
exposure apparatus, comprising:
(a) a first projection lens for focusing a first broad-band
alignment illumination and a second exposure illumination onto said
substrate located at said exposure plane of said exposure
apparatus;
(b) imaging optics responsive to said broad-band alignment
illumination from said alignment marks comprising:
(i) a second achromat lens in the path of light from said alignment
marks when illuminated by said first broad-band alignment
illumination;
(ii) a third refractive/diffractive hybrid lens in the path of
light from said second lens for correcting longitudinal and lateral
chromatic aberrations induced in said first projection lens by said
first broad-band alignment illumination; and
(iii) an aperture stop positioned between said second and third
lenses and adjacent to the surface of the refractive element of
said third refractive/diffractive hybrid lens;
(c) means for providing said first broad-band alignment
illumination and said second exposure illumination to said first
projection lens; and
(d) means for detecting the position of said alignment marks from
said imaging of said alignment marks by said first broad-band
alignment illumination.
2. The chromatic aberration-corrected optical system according to
claim 1, wherein:
said second achromat lens is located at the image point of said
first projection lens;
said second achromat lens images said first projection lens onto
said third refractive/diffractive hybrid lens; and
said third refractive/diffractive hybrid lens relays the image
formed by said first projection lens.
3. The chromatic aberration-corrected optical system according to
claim 1, wherein:
said chromatic aberration correction is achieved in accordance with
the following relationship: ##EQU10## where: .lambda. is a
wavelength within the wavelength range of said first broad-band
alignment illumination;
.lambda..sub.0 is a center wavelength within the wavelength range
of said first broad-band alignment illumination;
.phi..sub.1 is the optical power of said first projection lens;
.phi..sub.3 is the optical power of said third
refractive/diffractive hybrid lens;
B is the distance between said first projection lens and said
second achromat lens; and
C is the distance between said second achromat lens and said third
refractive/diffractive hybrid lens.
4. The chromatic aberration-corrected optical system according to
claim 1, in which said third refractive/diffractive hybrid lens
further comprises:
a refractive doublet lens having a first part and a second part;
and
a diffractive lens.
5. The chromatic aberration-corrected optical system according to
claim 4, wherein said third refractive/diffractive hybrid lens has
a total optical power in accordance with the following
relationship:
where:
.phi..sub.rA is the optical power of the first part of the
refractive doublet lens of the refractive/diffractive hybrid
lens;
.phi..sub.rB is the optical power of the second part of the
refractive doublet lens of the refractive/diffractive hybrid
lens;
.phi..sub.d is the optical power of the diffractive lens of the
refractive/diffractive hybrid lens; and
.lambda. is a wavelength within the wavelength range of the said
first broad-band alignment illumination.
6. The chromatic aberration-corrected optical system according to
claim 5, in which the spectral dispersion of the optical power of
the said third refractive/diffractive hybrid lens is determined in
accordance with the following relationships: ##EQU11## where:
##EQU12## is the Abbe-V number indicative of the spectral
dispersion of the first part of the refractive doublet lens;
and ##EQU13## is the Abbe-V number indicative of the spectral
dispersion of the second part of the refractive doublet lens;
and:
.DELTA..phi..sub.3 is the spectral dispersion of the total optical
power of the said third refractive/diffractive hybrid lens over a
wavelength range .DELTA..lambda..
.DELTA..lambda. is set to a value of 100 nm for the total
wavelength range of the said first broad-band alignment
illumination;
n.sub.rA ; n.sub.rB are the refractive indices of the first and
second parts, respectively, of the refractive doublet lens at
respective wavelengths .lambda..sub.0 =600 nm; .lambda..sub.1 =550
nm and .lambda..sub.2 =650 nm, with .DELTA..lambda.=(.lambda..sub.2
-.lambda..sub.1).
7. The chromatic aberration-corrected optical system according to
claim 6, in which the condition for achieving chromatic aberration
correction of the first and second pans, respectively, of the
refractive doublet lens of the said third refractive/diffractive
hybrid lens is determined in accordance with the following
relationship: ##EQU14##
8. The chromatic aberration-corrected optical system according to
claim 5, wherein the required optical power of the diffractive lens
of the said third refractive/diffractive hybrid lens is determined
in accordance with the following relationship: ##EQU15## where:
.phi..sub.d (.lambda..sub.0) is the optical power of the
diffractive lens at the center wavelength .lambda..sub.0 =600
nm;
.DELTA..lambda. is the wavelength range, set to a value of 100 nm,
of the said first broad-band alignment illumination; and
.DELTA..phi..sub.3 is the change in optical power of the said third
refractive/diffractive hybrid lens over the wavelength range
.DELTA..lambda..
9. The chromatic aberration-corrected optical system according to
claim 5, wherein the required total optical power of the refractive
doublet lens of the said third refractive/diffractive hybrid lens
is determined in accordance with the following relationship:
where:
.phi..sub.rA (.lambda..sub.0) is the optical power of the said
first part of the refractive doublet lens at a center wavelength
.lambda..sub.0 =600 nm;
.phi..sub.rB (.lambda..sub.0) is the optical power of said second
part of the refractive doublet lens at a center wavelength
.lambda..sub.0 =600 nm;
.phi..sub.3 (.lambda..sub.0) is the total optical power of the said
third refractive/diffractive hybrid lens at a center wavelength
.lambda..sub.0 =600 nm; and
.phi..sub.d (.lambda..sub.0) is the optical power of the
diffractive lens at a center wavelength .lambda..sub.0 =600 nm.
10. The chromatic aberration-corrected optical system according to
claim 9, wherein the required optical powers for the first and
second parts, respectively, of the refractive doublet lens of the
said third refractive/diffractive hybrid lens are established in
accordance with the following relationships: ##EQU16## where:
.phi..sub.rA is the optical power of the first part of the
refractive doublet lens;
.phi..sub.rB is the optical power of the second part of the
refractive doublet lens;
.phi..sub. (.lambda..sub.0) is the total optical power of the said
third refractive/diffractive hybrid lens at a center wavelength
.lambda..sub.0 =600 nm;
.phi..sub.d (.lambda..sub.0) is the optical power of the
diffractive lens at a center wavelength .lambda..sub.0 =600 nm;
and
V.sub.rA, V.sub.rB are the Abbe-V numbers indicative of the
spectral dispersion of the optical power of the first and second
parts, respectively, of the refractive doublet lens.
11. The chromatic aberration-corrected optical system according to
claim 10, in which the optical powers for the first and second
parts, respectively, of the refractive doublet lens of the said
third refractive/diffractive hybrid lens are determined in terms of
lens surface curvatures in accordance with the following
relationships:
and
where:
c.sub.1 ; c.sub.2 are the curvatures of the front and rear surface,
respectively, of the first part of the refractive doublet lens;
c.sub.3 is the curvature of the rear surface of the second part of
the refractive doublet lens; and
n.sub.rA (.lambda..sub.0); are the refractive indices of the first
and second part,
n.sub.rB (.lambda..sub.0); respectively, of the refractive doublet
lens, at a center wavelength .lambda..sub.0 =600 nm.
Description
FIELD AND SUMMARY OF THE INVENTION
The present invention relates generally to optical imaging systems
for broad-band imaging through a lens which is subject to chromatic
aberration. The invention is especially useful in exposure systems
and particularly in such systems as used in optical stepper
machines having an alignment detection system. The imaging system
uses a first projection lens, a second achromatic lens and a third
refractive/diffractive hybrid lens such that broad-band
illumination through the projection lens may be used to detect
alignment marks on a substrate with correction for chromatic
aberrations in the projection lens.
BACKGROUND OF THE INVENTION
An exposure apparatus known as a stepper is a main production
machine for semiconductor fabrication. A stepper is used to image
circuit patterns recorded on a mask and print them through a
projection lens on a wafer using a step and repeat procedure.
Generally, the circuit patterns are stratified into many layers. To
retain LSI (Large Scale Integrated) circuit performance, a given
layer must be accurately aligned to a previously patterned layer on
the wafer. Alignment is frequently accomplished by detecting
alignment marks on the wafer. Prior to exposure, the wafer is
usually coated with a photoresist-type material using a
spin-coating method. In the coating process, the photoresist
spreads radially, and often forms an asymmetrical shape around the
alignment marks, particularly when such marks are raised or
recessed with respect to surrounding, relatively planar areas on
the wafer or substrate. The intensity of light reflected from the
photoresist is sensitive to the thickness of the photoresist.
Therefore, images of the asymmetrically covered alignment marks can
be deformed asymmetrically; this produces degradation in the
alignment accuracy. However, it is well known that broad-band
illumination can reduce this problem. Using broad-band
illumination, the intensity of the reflected light is less
sensitive to fluctuation of the photoresist thickness, and
symmetrical images can be expected in spite of asymmetrical
photoresist coverage.
A broad-band light alignment system, which is separated from the
projection lens, i.e., a non-TTL (Through-The-Lens) system is
described by Nishi in U.S. Pat. No. 4,962,318, issued Oct. 9, 1990.
However, there are difficulties with Nishi's system. In practice,
the projection lens is mounted using structures of steel, and the
lens can move slightly due to environmental factors, such as
temperature variations or vibration. A non-TTL system cannot
respond to such movement of the projection lens, hence, there are
alignment errors. In contrast, a through-the-lens (TTL) alignment
system incorporates environmentally induced movements of the
projection lens assembly of the stepper automatically, a feature
which is essential for accurate alignment. There is, however, a
different problem with a TTL alignment system using broad-band
light. Normally, the projection lens is designed to have the best
performance at a single exposure wavelength (typically, an
ultraviolet wavelength). The wavelength of the light used for
alignment should be selected at a different wavelength from the
exposure light so as not to expose the photoresist. Therefore, TTL
systems using broad-band alignment light have difficulty generating
high quality images due to the longitudinal and latitudinal or
lateral chromatic aberration introduced by the projection lens. To
overcome this problem, a chromatic aberration correction system was
described by Komoriya et. al. in U.S. Pat. No. 5,094,539, issued
Mar. 10, 1992. The Komoriya et. al. correction system consists of
more than ten refractive lens elements, which is complicated and
expensive to fabricate and to maintain. Yoshitake et. al. describe
in JP4-019800, filed in Japan on Feb. 5, 1992, a reduction
projection exposure system comprising a reduction projection lens
optimized at one wavelength, an illumination apparatus illuminating
alignment marks on a substrate through the reduction projection
lens with broad-band light, and an alignment mark detection
apparatus having an optically diffractive holographic element
intended for correcting chromatic aberrations caused in the
reduction projection lens during the alignment mark detection.
However, the Yoshitake et. al. disclosure fails to provide any
lateral chromatic aberration correction, and provides only marginal
longitudinal chromatic aberration correction with the holographic
diffractive optical element in the alignment apparatus.
Furthermore, diffractive elements of the holographic type are
generally less effective transmitters of light than diffractive
elements of the kinoform type or of the blazed-grating type.
Highest possible light transmission is important in an alignment
mark detection system.
To counteract the spectral or wavelength dispersion introduced by
the projection lens, Komoriya et. al. had to employ many refractive
lens elements. The projection lens itself usually consists of more
than 10 glass elements, and each glass element introduces
dispersion, which causes chromatic aberration. Therefore, using a
conventional approach, to correct these aberrations, one
essentially needs to use a comparable number of refractive lenses
in the alignment system, as disclosed in Komoriya et. al. To reduce
the number of elements, at least one element in the correction
system must provide a large and negative dispersion value. Glass
elements cannot satisfy this requirement. However, a diffractive
lens has precisely these characteristics. The dispersion (or Abbe
V-number) of a solely diffractive lens element can be -3.45, while
that of any refractive glass lens element ranges between 20 and 80.
The V-number of a diffractive lens is approximately seven times
more dispersive than any known glass and exhibits a negative
dispersion, i.e., with a diffractive lens a red ray of light bends
more than a blue ray, whereas with a refractive glass lens blue
bends more than red. The operation of a diffractive lens relies on
interference and diffraction, rather than on refraction of light as
in a glass lens. Diffractive lenses include holographic optical
elements, blazed gratings and kinoforms. By utilizing a diffractive
lens, it is possible to significantly reduce the number of elements
necessary for the correction system. This reduction of the number
of elements has been disclosed by Yoshitake et. al. in the
aforementioned JP4-019800 patent application, although with
inadequate correction for chromatic aberrations.
OBJECTS AND ADVANTAGES OF THE INVENTION
In accordance with the present invention, there is provided a
refractive/diffractive hybrid optical alignment system for a
broad-band through-the-lens (TTL) optical imaging system,
especially for imaging alignment marks on a substrate in stepper
machines.
It is a principal object of the present invention to provide a TTL
alignment system which corrects for both longitudinal and lateral
chromatic aberrations originating in the reduction projection lens
of a stepper machine.
A further object of the invention is to provide a TTL alignment
system with a refractive/diffractive hybrid lens for chromatic
aberration correction throughout a wavelength range.
A still further object of the invention is to provide a TTL
alignment system which corrects for chromatic aberrations in a
reduction projection lens throughout a wavelength range of between
about 550 nm and about 650 nm of the broad-band alignment
illumination.
Another object of the invention is to provide a TTL alignment
system with chromatic aberrations correction having a reduced
number of refractive lens elements.
Still another object of the invention is to provide a broad-band
TTL optical alignment system in which only two achromatic
refractive lenses are used in addition to the
refractive/diffractive hybrid lens.
Chromatic aberrations, in longitudinal as well as latitudinal (or
lateral) directions, occur in refractive optical lens systems (for
example, in reduction projection lens assemblies of stepper
machines) whenever such lens systems are operated at illumination
wavelengths different from a single wavelength for which the
projection lens has been optimized. Chromatic aberrations are a
consequence of the spectrally dispersive nature of optical
elements. For example, a stepper reduction projection lens
optimized for operation at a single wavelength of about 436 nm (the
so-called g-line of a mercury lamp light source), or optimized for
operation at about 405 nm (h-line), or at about 365 nm (i-line), or
even in the far ultraviolet region at about 250 nm (an emission
wavelength of an excimer laser source), will have chromatic
aberrations when operated at other, non-optimized wavelengths.
Chromatic aberrations are particularly pronounced, when a
relatively broad spectral range (for example, a range from 550 nm
to 650 nm) of illumination is to be passed through a projection
lens optimized at, for example, 405.5 nm (h-line of a mercury lamp
light source), especially since the broad-band illumination has to
pass through-the-lens (TTL) twice,-namely once through the lens as
illuminating light for alignment marks in the direction toward the
substrate having alignment marks, and once as reflected light back
through the lens in the direction of the alignment mark detection
system. Such pronounced chromatic aberrations can seriously distort
and/or blur the appearance of alignment marks, and can therefore,
adversely affect the positional accuracy of alignment between marks
on the wafer substrate and marks on a mask projected onto said
substrate through the reduction projection lens of a stepper
machine.
Correction for chromatic aberrations, in accordance with the
present invention, is accomplished by an alignment system having a
hybrid (or compound) refractive/diffractive lens and an aperture
stop associated with that hybrid lens.
One advantage of the present invention is that the spectrally
broad-band alignment system corrects for both longitudinal and
lateral chromatic aberrations in a projection lens during the
alignment procedure.
Another advantage of this invention is that the broad-band
alignment system uses fewer refractive lens elements than prior art
systems.
A further advantage of the invention is that the spectrally
broad-band alignment system is a TTL alignment system.
A still further advantage of the invention is that the spectrally
broad-band alignment illumination is selected in a spectral
wavelength range where a photoresist coating, on a wafer or
substrate having alignment marks to be imaged, is not reactive to
that alignment illumination.
An additional advantage of the present invention is that the
chromatic aberration correction by the refractive/diffractive
hybrid lens can be used for TTL alignment in stepper machines
having a projection lens optimized for imaging exposure at any one
particular wavelength.
BRIEF DESCRIPTION OF THE DRAWINGS
The foregoing and other objects, features and advantages of the
invention will become more readily apparent by reference to the
following detailed description when considered in conjunction with
the accompanying drawings, wherein:
Fig. 1 is a schematic representation of a 3-lens model system used
to define the condition required for an achromatic optical
system;
FIG. 2 shows a practical configuration of an achromatic optical
system using the hybrid (or compound) refractive/diffractive lens
in accordance with the present invention;
FIG. 3 indicates schematically the spread of image points due to
chromatic aberration in an h-line stepper projection lens, caused
by broad-band through-the-lens (TTL) illumination from the
alignment system;
FIG. 4 shows the spectrally dependent (chromatic) change in image
height of the principal ray (LCA);
FIG. 5 is a schematic layout of an h-line (405.5 nm) projection
lens assembly and of the two alignment system lenses, in accordance
with the invention, and showing the lens surfaces and their
relative positions as well as the object and image planes;
FIG. 6 depicts the relationship between the location of an aperture
stop with respect to the surfaces of the lens elements of the
projection lens of FIG. 5, and image height of the principal ray
(LCA) of the projection lens assembly of FIG. 5;
FIG. 7 indicates the wavelength dependence of the optical power of
the projection lens assembly of FIG. 5;
FIG. 8 is a graph showing the calculated required optical power of
the third lens of the 3-lens model system of FIG. 1;
FIG. 9 shows a plot of the difference in image plane location
.DELTA.L at different wavelengths, relative to the image plane
location at the center wavelength (.lambda..sub.O =600 nm) versus
wavelength;
FIG. 10 indicates the difference in image magnification in percent
at different wavelengths, relative to the magnification at the
center wavelength (.lambda..sub.O =600 nm) versus wavelength;
and
FIG. 11 shows a currently preferred embodiment of the broad-band
illumination through-the-lens (TTL) alignment system for a stepper
machine in accordance with the present invention.
DETAILED DESCRIPTION
In FIG. 1, a so-called Shupmann 3-lens optical system is shown as a
model system to define the condition required for an achromatic
optical system, i.e., for an optical system free of chromatic
aberrations throughout a desired wavelength range of an
illuminating system. The Shupmann optical system has been analyzed
by Faklis and Morris, as reported in the publication Opt. Engin.,
No. 6, Vol. 28, 1989, pp. 592-598.
Lens 2 is located at the image point of lens 1 and also images lens
1 onto lens 3. Lens 3 relays the image formed by Lens 1. A
condition required for an achromatic system is given by the
following equation: ##EQU1## where .lambda. is an operating
wavelength, .lambda..sub.0 is the design (or center) wavelength in
a range of the operating wavelength, .sub..phi.1 is the optical
power of lens 1, .sub..phi.3 is the optical power of lens 3, B is
the distance between lens 1 and lens 2, and C is the distance
between lens 2 and lens 3.
FIG. 2 shows a practical Configuration of an achromatic optical
system using the hybrid (or compound) refractive/diffractive lens
in accordance with the invention. Lenses 1,2 and 3 in FIG. 1 have
been replaced by projection lens 11, the achromat lens 31 and
refractive/diffractive hybrid lens 33, respectively. A paraxial ray
81 of the center wavelength .lambda..sub.0 emitted from an
alignment mark on the wafer 12 converges on the front principal
point 31A of the achromat lens 31 through the projection lens 11,
and then, emitted from the rear principal point 31B, the paraxial
ray 81 converges on the secondary image plane 33A through the
refractive/diffractive hybrid lens 33. As an example for the
projection lens 11, an h-line (405.5 nm) projection lens which is
disclosed by Shafer in U.S. Pat. No. 4,770,477, is taken. The data
of the curvatures, thickness, and the glass materials in the
projection lens are listed in Table 1.
TABLE 1 ______________________________________ Mercury h-line
(405.5 nm) Design 5 .times. Magnification 0.35 numerical aperture
24 mm field diameter Unit; mm Surface Curvature Thickness Glass
______________________________________ Object 0.00000000 12.836980
Air S1 0.00920404 3.810000 Schott LF5 S2 0.01812579 30.649824 Air
S3 -0.02523014 6.350000 Fused Silica S4 -0.01903636 12.037365 Air
S5 -0.00902611 13.390880 Fused Silica S6 -0.01236653 4.795520 Air
S7 0.00285564 31.750000 Fused Silica S8 -0.01021591 2.540000 Air S9
0.00531074 35.560000 Fused Silica S10 -0.00915616 1.893291 Air S11
-0.00964527 8.321446 Schott LF5 S12 0.00775857 1.783080 Air S13
0.00762794 40.640000 Fused Silica S14 -0.00574017 2.540000 Air S15
0.00872893 31.750000 Fused Silica S16 -0.00162331 2.540000 Air S17
0.01518926 7.620000 Fused Silica S18 0.01815358 48.874223 Air S19
-0.01119420 7.620000 Schott LF5 S20 0.00558275 3.540000 Air Stop
0.00000000 15.151860 Air S22 -0.02072136 21.414740 Fused Silica S23
-0.01501763 3.169920 Air S24 -0.00302574 17.780000 Fused Silica S25
-0.00817112 2.540000 Air S26 0.00079734 15.240000 Fused Silica S27
-0.00336819 0.000000 S28 0.00331052 15.240000 Fused Silica S29
0.00042021 606.103632 Air Image 0.00000000 0.000000
______________________________________
To improve the performance at the h-line (exposure) wavelength, the
data given by Shafer had to be modified slightly.
In the design for the alignment system, the illumination
wavelengths were set to range from 550 nm to 650 nm. We have found
that this range is large enough to eliminate the sensitivity of the
alignment accuracy due to the fluctuation of photoresist thickness
[also see Nishi, U.S. Pat. No. 4,962,318, col. 3, lines 18-27].
Alignment marks are usually located at the edge of the field so
that the optical path of the alignment system does not interfere
with the exposure light. For an exemplary alignment system, the
object height is selected to be 10 mm, and it follows that the
system is an off-axis optical system. In this system each
wavelength produces a different image point. FIG. 3 shows the
spread of the image points of the h-line projection lens. The
numerals, 82A, 82B and 82C indicate the paraxial rays at 550 nm,
600 nm and 650 nm, respectively. The numerals 83A, 83B and 83C show
the image planes, and the symbols h.sub.A, h.sub.B and h.sub.C
indicate the image height for the wavelengths 550 nm, 600 nm and
650 nm, respectively. The image plane distances L.sub.AB, and
L.sub.BC, are 5.65 mm and 5.63 mm, respectively. The image
magnification difference .DELTA.M is defined as follows: ##EQU2##
.DELTA.M is 1.18% for the projection lens listed in Table 1.
Using thin-lens theory, the chromatic change in image height of the
principal ray of lens 1 in Fig. 1 is essentially zero. However, the
chromatic change in image height of the principal ray of an actual
projection lens is not zero. FIG. 4 shows the chromatic change in
image height of the principal ray, LCA. The numeral 84A, 84B and
84C indicate the principal rays of wavelength 550 nm, 600 nm and
650 nm, respectively. LCA denotes the height difference between the
rays at wavelengths 550 nm and 650 nm at the image plane, relative
to the center wavelength .lambda..sub.O =600 nm. LCA should be
urged to be zero to complete an achromatic Shupmann system. Using
the publication, Aberrations of Optical Systems, W. T. Welford,
Adam Hilger,1989, p. 207, it is found that LCA can be described by
the following equation: ##EQU3## where u'.sub.k is the convergence
angle of the marginal ray in the image space, n.sub.S and
.beta..sub.S are the refractive index and the angle of refraction
of the principal ray in the medium after surface S, and Y.sub.S is
the height of the marginal ray.
FIG. 5 shows a layout of the h-line projection lens. LCA can be
made to be zero by changing the location of the aperture stop. FIG.
6 shows the relation between the aperture stop location and LCA. It
should be noted that LCA is zero around surface 15. When the stop
is set 1.9 mm behind surface 14, LCA becomes exactly zero. In a
practical system, it is impossible to set the stop at such a
location because it will degrade the performance of the projection
lens. Instead, the same effect can be obtained (see FIG. 2) by
setting the stop 32 in contact with the refractive/diffractive lens
33. In FIG. 2, the numeral 11A indicates an actual stop of the
projection lens 11. The numeral 11B shows the location of the stop
that makes LCA to be zero, and the numeral 11C shows the image of
the stop 11B that is seen from the image side. The distance B in
Eq. (1) is defined as the distance between the location of the stop
image 11C and the front principal point 31A of the achromat lens
31. The distance C is defined as the distance between the rear
principal point 31B and the stop 32 of the refractive/diffractive
hybrid lens 33. The principal point 31A Of the achromat lens 31 is
located at the image point of the projection lens 11 at the center
wavelength (.lambda..sub.O =600 nm).
Once a focal length of the achromat lens 31 is determined, a
distance C is obtained using the following equation:
Where F.sub.2 is a focal length of the achromat lens 31, and is set
to F.sub.2 =100 mm. The distance B is taken to be 951.89 mm.
Substituting these parameters into Eq. (4), the distance C is found
to be C=111.74 mm.
To achieve an achromatic system, it is required that Eq. (1) be
satisfied. The wavelength dependence of the optical power of the
projection lens, .sub..phi.1 (.lambda.) is shown in FIG. 7. The
h-line projection lens is designed to be least sensitive to the
wavelength variations around the h-line (.lambda.=405.5 nm). The
range of the light (550 nm to 650 nm) used for the alignment system
is far enough from the h-line to produce an approximately linear
dependence of lens power .sub..phi.1 (.lambda.) versus
wavelength,.lambda.. In Eq. (1), .sub.100 3 (.lambda.) is set to
1/55 mm.sup.-1 so that the total magnification of the system will
be approximately 5/1. Substituting the above parameters into Eq.
(1), the required optical power for lens 3 (in FIG. 1 ), .sub.100 3
(.lambda.), is obtained. FIG. 8 shows a plot of this required lens
power, .sub.100 3 (.lambda.). Note that .sub.100 3 (.lambda.), is
also approximately linear due to the linearity of .phi..sub.1
(.lambda.) in the wavelength range between 550 nm and 650 nm. The
following solution is based on this linearity of optical powers
with wavelength. This solution is also applicable to projection
lenses designed for operation with an i-line (.lambda.=365 nm)
exposure wavelength or the excimer laser (e.g., KrF laser,
.lambda.=248 nm) illumination because these lenses are also
optimized at the exposure wavelength and their relations between
the power and the wavelength are also linear in the range of
visible light used for the alignment system.
To obtain the best fit to the theoretical curve shown in FIG. 8,
the alignment system of the present invention uses a
refractive/diffractive hybrid lens 33. The refractive/diffractive
hybrid lens 33 in FIGS. 2 and 11 consists of a diffractive lens.
Using thin-lens theory, the total power of the
refractive/diffractive hybrid lens 33 is given by
where .phi..sub.rA is the power of the first part of the refractive
doublet, .phi..sub.rB is the power of the second part of the
refractive doublet, and .phi..sub.d is the power of the diffractive
lens. The first derivative of Eq. (5) is given by ##EQU4## where
.lambda..sub.0, .lambda..sub.1 and .lambda..sub.2 are 600 nm, 550
nm and 650 nm, respectively, and .DELTA..lambda. is 100 nm. To
obtain the best fit to the theoretical curve shown in FIG. 8, the
following condition is required: ##EQU5##
This is the condition required if the refractive doublet is to be
an achromat. Using Eqs. (6) and (7), the required optical power of
the diffractive lens is found to be: ##EQU6## The total optical
power of the doublet is
The required optical powers for each component of the doublet are
obtained using Eqs. (7) and (9) as follows: ##EQU7##
On the other hand, these optical powers can be described in terms
of surface curvatures:
where c.sub.1 and c.sub.2 are the curvatures of the front and the
rear surface of the first part of the doublet, and c.sub.3 is a
curvature of the rear surface of the second part of the doublet. To
make it easy to form a diffractive lens on the doublet, curvature
c.sub.3 is set to zero (i.e., required to be a flat surface). Once
.phi..sub.d (.lambda.)is calculated using Eq. (8), c.sub.2 is
obtained with Eqs. (11) and (13), and cl is calculated using Eqs.
(10) and (12). For this design example, we choose the following
glass types for the doublet-Schott BK7 and F2, which are
inexpensive glass types commonly used in industry. Using the design
procedure described above, the first-order design parameters for
the refractive/diffractive hybrid lens 33 are obtained as follows:
.phi..sub.d (.lambda..sub.0)=0.0075 mm.sup.-1, c.sub.1 =0.0255
mm.sup.-1, c.sub.1 =0.0255 mm.sup.-1, c.sub.2 =-mm.sup.mm.sup.-1
and c.sub.3 =0.0 mm.sup.-1.
These parameters and the distance C are optimized utilizing
commercial lens design software. Table 2 shows the lens data
obtained after the optimization.
TABLE 2 ______________________________________ Broad-band (550
nm-650 nm) Alignment System Design 5 .times. Magnification 0.175
numerical aperture Object height; 10 mm Unit; mm Surface Curvature
Thickness Glass ______________________________________ Object
0.00000000 12.836980 Air S1 0.00920404 3.810000 Schott LF5 S2
0.01812579 30.649824 Air S3 -0.02523014 6.350000 Fused Silica S4
-0.01903636 12.037365 Air S5 -0.00902611 13.390880 Fused Silica S6
-0.01236653 4.795520 Air S7 0.00285564 31.750000 Fused Silica S8
-0.01021591 2.540000 Air S9 0.00531074 35.560000 Fused Silica S10
-0.00915616 1.893291 Air S11 -0.00964527 8.321446 Schott LF5 S12
0.00775857 1.783080 Air S13 0.00762794 40.640000 Fused Silica S14
-0.00574017 2.540000 Air S15 0.00872893 31.750000 Fused Silica S16
-0.00162331 2.540000 Air S17 0.01518926 7.620000 Fused Silica S18
0.01815358 48.874223 Air S19 -0.01119420 7.620000 Schott LF5 S20
0.00558275 3.540000 Air S21 0.00000000 15.151860 Air S22
-0.02072136 21.414740 Fused Silica S23 -0.01501763 3.169920 Air S24
-0.00302574 17.780000 Fused Silica S25 -0.00817112 2.540000 Air S26
0.00079734 15.240000 Fused Silica S27 -0.00336819 0.000000 S28
0.00331052 15.240000 Fused Silica S29 0.00042021 622.440000 Air S30
0.01555694 6.230000 Schott SK11 S31 -0.02223210 2.500000 Schott SF5
S32 - 0.00554232 100.473524 Air Stop 0.03318571 5.300000 Schott SK7
S34 -0.04330520 4.000000 Schott F2 S35 0.00000000 0.000100 Virtual
S36 -0.00000063 103.095781 Air Image 0.00000000 0.000000
______________________________________
For the optimization with commercial lens-design programs, it is
convenient to describe the diffractive lens in the same manner as a
refractive lens. Sweatt proposed a convenient method to describe a
diffractive lens for use with lens-design software in his paper,
"Describing holographic optical elements as lenses", J. Opt. Soc.
Am., Vol. 67, No. 6, 1977, pp. 803-808. With this method, the
optical power of a diffractive lens, .phi..sub.d (.lambda.) can be
described as a thin lens that possesses a very large index of
refraction, i.e.,
in which, ##EQU8## where c.sub.A and c.sub.B are the curvatures of
a diffractive lens, and n(.lambda.) is an effective refractive
index of the diffractive lens. To accurately model the behavior of
a diffractive lens, N should be a very large number. In practice, N
is typically set to N=10,000. Using this method, a diffractive lens
can be described just like a refractive lens as shown in Table 2.
The curvature, c.sub.A which corresponds to the curvature of S35 is
set to zero for ease of fabrication.
FIG. 9 shows a plot of the difference in image plane location
.DELTA.L relative to the location of the image plane using the
center wavelength (.lambda..sub.0 =600 nm) versus wavelength, and
FIG. 10 shows the relationship of the image magnification
difference at different wavelengths, respectively. Note that in the
region from 550 nm to 650 nm, the spread of .DELTA.L and image
magnification differences are less than 50 .mu.m, and less than
0.05%, respectively. Originally, without the correction system,
these numbers are 11.28 nm and 1.18%, respectively.
Once .phi..sub.d (.lambda..sub.0)is obtained, a blaze profile of a
diffractive lens surface, d(r) can be calculated as follows:
##EQU9## where m is an integer >0, and n.sub.dif is a refractive
index of an actual material of the diffractive lens. The blaze
profile can also be approximated by a linear profile or step or
saw-tooth like structures.
FIG. 11 shows the currently preferred embodiment of the alignment
system for a stepper machine, in accordance with-the present
invention. The exposure system comprises the mask 10, the
projection lens 11 and the wafer or substrate 12. Circuit patterns
on the mask 10 are printed onto the wafer 12 through the projection
lens 11 with the single-wavelength exposure light 13. The alignment
system comprises all the illustrated parts except for the mask 10.
The alignment system, shown in FIG. 11, measures the alignment mark
location in the perpendicular direction to the plane of the paper
(i.e., in the latitudinal or lateral direction). An alignment
system to measure in the parallel direction (i.e., the longitudinal
direction) to the plane of the paper is omitted in FIG. 11 for
purposes of clarity. The illumination light for alignment is
emitted from a halogen light source 21, and the light is guided by
the optical fiber 22. The achromat lens 23 conjugates the end of
the optical fiber 22 and the entrance pupil 11AA of the projection
lens 11. The light emitted from the optical fiber 22 passes through
achromat lens 23. The designed wavelength region, i.e., from 550 nm
to 650 nm is selected by optical bandpass filter 24. The light
passing the bandpass filter 24 is bent by the bending mirror 25 and
impinges onto pelicle beam splitter 26. The light bent by the
pelicle beam splitter 26 is bent again by the bending mirror 27
and, passing the center of the entrance pupil 11AA of projection
lens assembly 11, illuminates the alignment mark 12A on wafer 12.
The reflected light from the alignment mark 12A goes back through
the projection lens 11, the bending mirror 27 and the pelicle beam
splitter 26, and converges at the achromat lens 31. The first image
of the alignment mark 12A is obtained in the achromat lens 31. The
light passing achromat lens 31 diverges into the aperture stop 32
and the refractive/diffractive hybrid lens 33, and converges again
at the secondary imaging plane 33A.
The secondary image of the alignment mark 12A is magnified and
imaged by objective lens 34 onto TV camera 35. The final magnified
image taken by TV camera 35 is transferred to a A/D converter 41,
and the image 42A is processed by the computer 42 to measure the
location of the alignment mark 12A on wafer or substrate 12.
From the foregoing detailed description of the preferred
embodiments of the invention, it will be apparent that there has
been provided a chromatic aberration-corrected broad-band
through-the-lens (TTL) optical imaging system for imaging alignment
marks deposed on a substrate or wafer located in the exposure
imaging position of a projection lens of an optical stepper
machine. Variations and modifications of the herein described
optical system within the scope of the invention, will undoubtedly
suggest themselves to those skilled in the art. Accordingly, the
foregoing description is to be taken as illustrative and not in a
limiting sense.
* * * * *